Eighteen of 31 trucks carrying medicine, wheat flour and clothing were destroyed in the town of Orum al-Kubra. Between 12 and 20 people were killed.

The UN and Syrian Arab Red Crescent said the aid mission was authorized by the Syrian government, and both U.S. and American coalition forces were informed.

The U.S. and aid agencies said the convoy was hit by an airstrike. Russia’s Defense Ministry said no Russian or Syrian aircraft carried out strikes. The ministry also released drone footage showing militants in a pickup truck with a mortar gun accompanying the aid convoy.

The attack drew outrage from UN officials, as the UN suspends all aid convoys in Syria.

The cause of the attack remains unclear. Also unknown is who carried out the attack.

What’s clear is that this has brought a week-old U.S.-Russia brokered truce nearer to the brink of collapse, prompting U.S. Secretary of State John Kerry to assure diplomats at Tuesday’s Syria meeting that the ceasefire is not dead.

But that ceasefire appears all but dead, at least for now. Clashes are intensifying in Syria, U.S. and Russia have bickered over releasing details of their ceasefire deal and over a mistaken U.S. airstrike on Syrian soldiers, and now finger-pointing is dominating the aftermath of Monday’s attack. Syria’s fate remains as uncertain as ever.